VPN: a WireGuard tunnel (#8)

The device joins a WireGuard network over whatever Wi-Fi it is on: one
peer, IPv4. A client's .conf is imported from the card (/vpn/wg0.conf) and
kept in the device's settings, private key included, never shown; Settings
offers to delete the file. A switch brings the tunnel up until the next
restart, "Start with Wi-Fi" every time; it waits for the clock, which a
handshake needs. VPN shows in the Status Bar.

The protocol is esphome/wireguard 0.4.8. It calls lwIP without lwIP's lock,
which this framework checks: every call into it is made with the lock held.

What goes through the tunnel is everything (AllowedIPs 0.0.0.0/0) or the
one subnet the device's tunnel address is in: lwIP routes by an
interface's subnet or by default, nothing finer. The import says how many
ranges it can't reach.

Checked against a test peer in both directions and against a real server,
with a configuration uploaded from a phone (docs/milestones/N1.md).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/ui/widgets.cpp

@@ -48,6 +48,11 @@ void statusBar(Canvas& c, const StatusInfo& info) {
         case StatusInfo::Wifi::Monitoring: right("MON", kAccent); break;
         case StatusInfo::Wifi::None: break;
     }
+    switch (info.vpn) {  // Q252: there while the tunnel is wanted, bright once the peer has answered
+        case StatusInfo::Vpn::Trying: right("VPN", kMuted); break;
+        case StatusInfo::Vpn::Up: right("VPN", kAccent); break;
+        case StatusInfo::Vpn::Off: break;
+    }
     switch (info.debug) {  // Q190: there while the console listens, bright with someone connected
         case StatusInfo::Debug::On: right("DBG", kMuted); break;
         case StatusInfo::Debug::Client: right("DBG", kAccent); break;